The processing of materials by means of microwave energy radiated within an enclosure has been wide spread in the home and industry for a number of years. The molecular agitation within the material resulting from its exposure to microwave energy provides frictional heat to cook or dry the material in a relatively short time period as compared to conventional gas or electric heating. The generators most frequently employed in such application are magnetrons and high power vacuum triodes.
The common frequencies of operation allocated by government agencies for microwave heating systems are centered at 915 and 2450 megahertz. The intensity of microwave energy permitted to leak from domestic and/or industrial microwave heating systems is restricted to less than 10 milliwatts per square centimeter. For example, in the United States, the Department of Health Education and Welfare presently requires that the microwave energy leakage from a domestic oven must not exceed one milliwatt per square centimeter in the factory or five milliwatts per square centimeter in the home. The Occupational Safety and Heath Administration requires a microwave energy exposure of less than ten milliwatts per square centimeter. The standard adopted by the International Microwave Power Institute for intensity of microwave radiation leakage from domestic and industrial systems is "less than ten milliwatts per square centimeter".
Systems employing the use of microwave energy for heating or drying must have heretofore prevented the microwave energy from escaping the enclosure where the product is processed. For example, microwave energy leakage has been prevented in a batch system where the product is delievered into the enclosure through a door by closing and sealing the door during operation. However, in many industrial microwave heating applications it is desirable that access apertures in the microwave energy processing region remain open during operation so that a transporting system such as a conveyor belt can continuously move products through said processing region. The prevention of microwave energy leakage through these access apertures presents a serious problem for high volume microwave processing systems.
Panels made from lossy materials have been used to line the inner walls of a hollow structure, or tunnel, which has one end abutting an aperture. The product then passes through the hollow structure, or tunnel, on a conveyorized system and is presented to the microwave processing region. Microwave energy which radiates down the tunnel toward the exterior of the system is absorbed by the lossy material. However, for large apertures, the system efficiency is relatively low because a substantial amount of energy is consumed by the lossy material. Also, if mutually orthogonal dimensions of a cross-section of the tunnel are large relative to a free space wavelength of the microwave energy, the tunnel has to be prohibitively long to be an effective seal.
A means of improving sealing when the mutually orthogonal sealing tunnel dimensions are increased uses a plurality of thin metal flaps hung in the tunnel having lossy walls. As the product passes through, it pushes the flaps aside. However, such flaps are not a sufficiently effective seal when the tunnel cross-sectional mutually orthogonal dimensions are substantially greater then a free space wavelength of the microwave energy or when product pushing aside the flaps is not sufficiently lossy. Also the increased surface area of such enlarged tunnel accounts for additional microwave energy loss from the processing region further reducing the system efficiency.